This invention relates to a servo-power-assisted hydraulic brake-actuating arrangement comprising a brake valve for controlling servo pressure which is proportional to the actuating force, and a first and second displacement piston, each defining a static fluid circuit, with the second displacement piston being actuated on its rear end by the servo pressure introduced into a chamber behind the second displacement piston under control of the brake valve.
Such a brake-actuating arrangement is known from the article "Hydraulic Brake Boosting and Anti-Skid Control for Automotive Vehicles" which appeared on pages 375 to 381 of the journal "Automobiltechnische Zeitschrift" 75 (1973) 10. In the brake-actuating arrangement known from Figure 8 of the above-cited article, a brake valve controls servo pressure which is proportional to the force the vehicle operator applies to the brake petal, by taking fluid from a fluid accumulator. The servo pressure is used for actuating two displacement pistons each of which defines a static fluid circuit, so that a brake pressure proportional to the servo pressure is generated in each fluid circuit.
In addition, in this known brake-actuating arrangement, one of the two displacement pistons is mechanically actuatable by the vehicle operator directly if the accumulator pressure fails, without causing substantial loss of pedal travel. If such a case occurs, brake pressure builds up only in one of the two fluid circuits. This fluid circuit is preferably connected to the wheel brakes of the front axle which provides, as a rule, the higher braking force.
The fact that the known brake-actuating arrangement deliberately does not make use of an emergency activation of the second fluid circuit is based on the realization that it is not possible in heavy vehicles to build up in both fluid circuits a pressure sufficiently high to achieve a full braking effect by means of the pedal force alone, thereby obviating the need for an emergency activation of both fluid circuits.
However, when using servo-power-assisted hydraulic brake-actuating arrangements in medium-heavy to light vehicles, the force the driver is able to apply would suffice to build up a relatively high pressure in both brake circuits, the more so since, as a rule, the driver will depress the pedal with considerably more effort than usual, reacting with shock due to the reduced brake performance. However, this may result in an excessively high pressure being built up in the still actuatable fluid circuit which causes overbraking of the associated vehicle axle. Thus, in the event of servo power boost being inoperative, the brake performance will also in light vehicles be limited to a value achievable by but one vehicle axle, although the force the driver is able to exert on the pedal would permit a stronger braking to be achieved.
Practically the same applies if a heavy vehicle is braked on a road surface having a very low coefficient of friction, for instance, on an icy road. Also in this case it will be more advantageous if pressure, though relatively low, is built up in both fluid circuits so that both axles of the vehicle are slowed down. If pressure were built up in but one fluid circuit which would act on one axle only, the pressure, though being higher, could not be made full use of to achieve an optimum braking of the vehicle due to the low coefficient of friction.
In the special case where the vehicle is braked on a surface having a low coefficient of friction it is, however, essential that the brake force continue to be distributed, according to the requirements of the vehicle, among the two vehicle axles even during emergency braking conditions in the absence of servo-power boost to ensure an optimum braking under the prevailing conditions.
However, from this result particular difficulties if in a brake-actuating arrangement of the type initially referred to an emergency braking is to be provided for both brake circuits. Namely, it is not possible in this system to just rigidly connect both displacement pistons with the brake pedal when the servo force has failed. The necessary brake-effort distribution would not be ensured under emergency braking conditions. To ensure this, means would have to be provided to achieve distribution of the driver's pedal effort among the two displacement pistons in accordance with the required distribution ratio. The provision of such means, however, entails a further difficulty in that only part of the actuating travel is to be made use of with the servo power available, whereas the complete travel is to be utilized when the servo force has failed. If in the design of such a brake-actuating arrangement all these requirements were fulfilled in an obvious manner, provided such fulfillment is at all possible, an extremely complicated structure would result which would substantially increase the risk that this brake-actuating arrangement is prone to malfunction. Besides, such a brake-actuating arrangement would hardly allow economy of manufacture.